中国南方二叠纪Ⅰ型层序界面沉积记录及事件分析

SEDIMENTS ON BOUNDARY SURFACES OF TYPE I DEPOSITIONAL SEQUENCE IN THE PERMIAN OF SOUTH CHINA

中国南方二叠纪 11个层序中的S1、S6、S7和S11四个Ⅰ型层序底界面具有特征沉积记录 ,反映在岩相、地球化学和生物演化方面 ,尤其是作为与海平面下降特征相对应的古风化壳、古喀斯特标志风化残积硅铝 /铁铝粘土层、铝质粘土层、残积角砾灰岩或淡水胶结等现象广为发育。二叠纪各相关Ⅰ型层序底界面的分布及其之上的火山沉积、沉积背景转变等具有重要的事件意义。研究结果表明 :S1底界面对应的是构造事件 ,即范围较广的黔桂运动 ;S7底界面反映的构造事件为东吴运动 ;S6和S11的底界面均代表海平面下降事件。中国南方二叠纪的层序地层划分及其性质不能用一种方案和一种模式套用。

Four type Ⅰ depositional sequences were recognized from the eleven depositional sequences of the Permian in South China by former researchers. In this paper, we will describe the sediments on the basal boundary surfaces of type Ⅰ sequences and use the data about the lithofacies, geochemistry and evolution of organisms to analyze the relationships between the sediments and geological events. Study has found that native soils of ferralitic and/or bauxite clays, residual breccia of limestone with freshwater cements, paleosol, and paleokarst are extensively distributed on these boundary surfaces in different type Ⅰ sequences. The trace elements, such as B, V/Ni, Ga, and the stable isotopes of δ 13 C and 87 Sr/ 86 Sr changed obviously across the boundary surfaces. There are also apparent differences between the fossil assemblages, such as fusulinids, ammonites, brachiopods, conodonts existing at the end of previous HSTs and the beginning of later TSTs, suggesting that the extinction and recovery of marine invertebrates probably happened during the times represented by the gaps of type Ⅰ boundary surfaces. Volcanic sediments have been observed in some places on the basal boundary surface of S 7. The sediments on the boundary surfaces of the type Ⅰ depositional sequences have important significance for understanding the related geological events. It has been demonstrated that sediments on the basal boundary surfaces of S 1 and S 7 probably were associated with the tectonic rising event, which was accompanied by some volcanic activity in the beginning of S 7 and controlled the falling of relative sea level. The sediments on the bases of S 6 and S 11 could be attributed primarily to the large scale falling of sea level. Based on results of this study, we believe that the definition and content of the Chuanqian movement, which was first named in Guizhou-Guangxi provinces, Southwest China and could be recognized between S 6 and S 7, should be revised. We also suggest that different classification of depositional sequence should be used for different sedimentary basins of the Permian in South China. For example, the S 1 is a type Ⅰ depositional sequence in the Permian of the Yangtze craton, but it could become a type Ⅱ depositional sequence of the same age in the Cathayan craton.